Recording materials of the said type are known. As light-sensitive component, their radiation-sensitive layer preferably comprises diazonium salts. These are usually products of the condensation of aromatic diazonium salts (for example diphenylamine-4-diazonium salts) and a carbonyl component (usually formaldehyde).
Refinements of these systems are moving towards increased light sensitivity or an improvement in run stability. The balance between the developability of the recording material after imagewise irradiation (rapid solubility of the components, good developer life) and the resistance of the finished offset printing plate in printing (achievable run size, sensitivity to chemicals, ink take-up) should be preserved here.
Printing plates having good printing properties, in particular having high run stability, are obtained, for example, if the binder in the layer has a high glass transition temperature. Recording materials which can be developed particularly easily and quickly using aqueous-alkaline developers comprise, as binder, polymers based on polyvinylbutyrals which have been modified with carboxyl groups (EP-A 152 819=U.S. Pat. No. 4,631,245), on polyurethanes which have been modified with carboxyl groups (EP-A 563 990) or on polyacrylates which contain corresponding proportions of acrylic or methacrylic acid units (DE-A 35 03 115). The said binders are relatively polar owing to their content of carboxyl groups, which means that the entire layer has polar properties. This in turn frequently results in the printing ink being taken up less well during printing or in an interaction occurring between the printing chemicals and the layer. Both are of course undesired.
Besides binders containing carboxyl groups, those containing sulphonate groups are also known. Ion pairs then form between the sulphonate groups of the binder and the diazonium groups of the light-sensitive component. Recording materials in which use is made of this interaction are described, for example, in WO 96/10217 and WO 98/34158. It is disadvantageous here that two polyelectrolytes of opposite charge have to be brought together, which may under certain circumstances result in complete precipitation.
The light sensitivity can be increased by chemical modification of the diazonium salt. Thus, the type of anions can be varied. Functional groups can be introduced, or other condensable carbonyl compounds can be used for their preparation. Thus, diazonium sulphonates are less soluble in water than diazonium chlorides or bromides and at the same time effect increased light sensitivity of the layer produced therewith (U.S. Pat. No. 3,867,147). Polymeric diazo resins containing relatively large non-polar bridging groups result in layers having increased light sensitivity. The printing plates produced therewith at the same time exhibit good printing properties, in particular good ink take-up, and achieve fairly long print runs. However, there is a negative correlation between the light sensitivity and the developability, i.e. the solubility of the diazo resins in the developer. Diazonium compounds having relatively large anions (organic sulphonates, etc.) or relatively large bridging groups between the individual diazonium groups have increased light sensitivity, but significantly worse solubility in conventional aqueous-alkaline developers. As a consequence, the developer becomes exhausted more quickly and has to be replaced more often.
In order to improve the solubility of the diazonium resins in the developer, anionic, cationic or neutral surfactants have been added to the layer. The surfactants increase the solubility of the layer as a whole, i.e. they improve the so-called basic solubility. However, they also act as plasticizers, meaning that the printing areas in the finished printing plate are less resistant. As a consequence, the achievable print run drops. In addition, the surfactants do not mix with the surrounding matrix to an adequate extent in some cases. They then accumulate at the layer surface, where they impair ink take-up during printing.
The surfactants are generally those which may also be present in developer formulations. Thus, for example, sulphonate surfactants present in the developer may interact with the diazonium salt of the layer and thus promote developability of the layers. However, surfactants in the developer are less effective than those in the layer, since they first have to diffuse into the layer in order to develop their action. A certain time span is necessary for this.
Certain coloured pigment dispersions are likewise suitable for increasing the solubility of the layer. These dispersions generally comprise soluble binders in the developer (EP-A 778 497 and EP-A 1 043 627). By contrast, soluble dyes are not very advantageous owing to their colouring and bleeding behaviour.